The United States on Tuesday warned Syrian President Bashar al-Assad that he will never secure a full victory and must reach a political compromise, on the eve of stinging new sanctions.

The war-torn nation has seen an intensifying economic crisis and on Wednesday the Caesar Act takes effect, a US law that targets companies that deal with Damascus.

Kelly Craft, the US ambassador to the United Nations, urged Assad to embrace Security Council Resolution 2254 of 2015 -- which called for a ceasefire, elections and political transition in Syria -- and talks under UN envoy Geir Pedersen.

"The Assad regime has a clear choice to make: pursue the political path established in Resolution 2254, or leave the United States with no other choice but to continue withholding reconstruction funding and impose sanctions against the regime and its financial backers," Craft told the Security Council.

"The Trump administration will take decisive steps to prevent the Assad regime from securing a military victory, and to steer the regime and its allies back toward special envoy Pedersen and the UN-led political process," she said.

Assad, backed by Russia and Iran, has succeeded in winning back virtually all of Syria except the Idlib area after a nine-year war that has killed more than 380,000 people.

The Caesar Act, passed by the US Congress last year with bipartisan support, seeks to prevent Assad's normalization without accountability for human rights abuses.

The law penalizes in the United States any company that deals with Assad and blocks reconstruction assistance from Washington.

The law is named after a former Syrian military photographer who fled at great personal risk in 2014 with 55,000 images of brutality in Assad's jails since he launched his crackdown on protests three years earlier.

With Syrian and Russian forces mounting a major offensive in Idlib, a UN-backed constitutional review aimed at reaching a peaceful solution has made little progress.

Pedersen told the Security Council that he was willing to resume constitutional talks in Geneva in late August.